The present invention relates to a process for preparing raw material powders or a raw material dispersion which is employed in the manufacture of reaction layers of a gas permeable electrode for use in a fuel cell, a secondary battery, an electrochemical reactor and the like.
A gas permeable electrode generally comprises a hydrophilic reaction layer and a hydrophobic gas permeable layer attached to each other, and a collecting member attached to the opposite surface of the gas permeable layer.
The hydrophilic reaction layer of the gas permeable electrode may comprise hydrophilic carbon blacks, polytetrafluoroethylene (hereinafter referred to as PTFE) and hydrophobic carbon blacks, and contains supported catalysts if necessary. In order to conventionally prepare a raw material dispersion for the reaction layer, the mixture of hydrophobic (or hydrophilic) carbon blacks, PTFE, water and a surface-active agent is filtered to form a cake, which is then thermally dried and pulverised.
In such a process for preparing the raw material powders, uniform and fine particles can not be obtained because the dried cake is mechanically pulverised. Even if the thus obtained powders are mixed with hydrophilic (or hydrophobic) carbon blacks or hydrophilic carbon blacks supporting catalysts and PTFE to form the reaction layer by means of calcination, hydrophilic portions and hydrophobic portions may not be uniformly and finely dispersed, to lower the catalytic performance because the raw material powders are non-uniform and bulky.
Further, in the gas permeable electrode which has been prepared from the above mixed powders containing the catalysts, the catalysts supported on the hydrophilic carbon blacks may be liberated to aggregate, or the catalysts among the PTFE fine powders and the hydrophobic carbon blacks may flow to aggregate in a relatively short period of use. Consequently, the dispersion density of the catalysts comes to be less uniform and the clusters of the catalysts become more bulky. When an electrolyte penetrates and gas disperses and permeates, they are in contact with only the surface of the aggregated catalysts and not in contact with the internal catalysts so that less catalysts can act effectively. Accordingly, the quantity of current flow per unit area is so low that the effectiveness is quite unsatisfactory.
The degradation of the catalysts due to the liberation and the flowing is rapid, and the life of the gas permeable electrode may be shorter.